1. Field of the Invention
The present invention relates to articulated vehicles and more particularly relates to steerable articulated automotive vehicles which are frequently operated off the highway over rugged terrain.
2. The Prior Art
Various constructions of steerable articulated automotive vehicles have been proposed by the prior art, many of which have been employed with farming tractors or similar vehicles which must be operable over rugged terrain.
When an articulated vehicle is intended for use off of the highway, its construction must be such that the ground engaging wheels, or treads, supporting one vehicle section can move vertically relative to the remaining vehicle wheels or treads in such a way that traction is maintained when the vehicle moves along rough ground. Various constructions have been proposed to provide this feature.
According to some proposals, a two section vehicle frame was provided with an articulating joint between the frame sections which permitted the frame sections to be provided relatively about a vertical steering axis. Wheel axles supporting one or both frame sections were oscillatable relative to the longitudinal mid-line of the associated frame section when uneven surfaces were encountered. These proposals generally necessitated large, heavy and complex frames and suspensions.
Another type of proposal employed a frame section articulating linkage which permitted articulation of the frame sections both about a vertical steering axis and a longitudinal axis. These articulating linkages were generally complex gimble-like constructions and when employed in heavy duty vehicles they had to be of great strength. In some proposals the steering forces were applied to parts of the gimbal structure which necessitated a construction capable of transmitting extremely large steering torques.
A more recent approach provided a frame section articulating linkage which included a universal spherical bearing assembly between the frame sections and a single floating link separately connected between the frame sections to limit the action of the spherical bearing assembly. One or more hydraulic steering rams were connected between the frame sections to provide for steering the vehicle by pivoting the frame sections about the steering axis.
In vehicles where a single ram was employed on one side of the longitudinal midline of the vehicle, the oscillation of one frame section relative to the other when the vehicle was being driven in a desired direction caused the steering angle between the frame sections to change, i.e., the steering geometry of the vehicle was altered, and the vehicle changed direction. This was due to the fact that when one frame section oscillated unrestrained (i.e., in the absence of a steering ram) relative to the other frame section, no two points of the respective frame sections on the same side of the vehicle mid-line remained the same distance apart. However when any such points were connected by a steering ram, the distance between the points could not change when the ram was not extended or retracted and as a consequence the vehicle was turned when relative frame oscillations occurred.
Where two steering rams were connected between the frame sections, one on each side of the vehicle mid-line, and the frame sections oscillated relative to each other, the rams, their associated hydraulic systems, and the connections between the rams and the respective frame sections, were frequently subjected to large magnitude compressive or tensile forces which tended to damage the components of the steering system and/or cause the introduction of air or vapor bubbles into the hydraulic fluid. Where large magnitude forces were attempted to be controlled by the use of hydraulic fluid pressure relief valves, the operation of the relief valves tended to cause abrupt turning of the vehicle while the steering wheel remained stationary.